In oil and gas wells not all the oil and gas in a reservoir can be recovered. This problem is particularly severe in multilayer reservoirs. Using traditional production methods oil and gas may be bypassed due to the cross flows or water coning that occurs as the layers in the reservoirs are depleted.
Production of oil and gas from a reservoir is dependant on the mobility of the fluid in the reservoir. Mobility is effectively the ease with which fluids can be produced from a formation and depends on characteristics such as porosity, permeability and fluid viscosity. In multilayer reservoirs, for a given fluid viscosity, the more permeable layers will typically have a higher production rate and will be depleted much faster than the less permeable layers in the reservoir. If one layer is depleted faster than the other layers, this can lead to the build up of greater pressure differences between the layers than are initially present. These pressure differences can cause cross-flows between the layers and the greater pressure gradient formed during production can cause water coning and water breakthrough in the wellbore. To prevent cross-flow and early water breakthrough the production parameters from the various layers have to be closely controlled.
In a conventional well completion, a single borehole is drilled through the layers of the reservoir. The borehole is usually lined with a steel casing surrounded by cement which prevents fluid communication between the layers. Communication between the layers and the interior of the casing is permitted by forming perforations in the casing and cement. The number and arrangement of perforations in a layer will determine the degree of communication possible between the layer and the borehole.
In certain circumstances, no casing is used (barefoot completion) but such completions can be problematic due to the lack of support for the formation and the possibility of borehole collapse, sanding and the like. Ultimately, the ability of the borehole to produce fluids is limited by the surface area of the borehole in the layer in question.
It has been proposed to complete a borehole and manage production from the different layers to reduce the pressure differences between the various layers and so minimise the problems indicated above. However, this can often lead to an overall reduction in the rate of production from the borehole.
In order to improve drainage from a reservoir, it has been proposed to drill lateral boreholes or drain holes, which extend from the main borehole into the producing reservoir layer. While such an approach does allow a greater producing surface to be obtained in a given reservoir layer, the problems of pressure differences between layers still occur.
Therefore it is an object of the invention to provide a method which allows construction of lateral boreholes which can flatten the pressure profile of multilayer reservoirs to enhance the recovery of hydrocarbons.